1. Field of the Invention
The present invention relates to a thin, light-weight organic electroluminescence device which has excellent durability and is impact-resistant.
2. Related Background Art
Display devices using electroluminescence (hereinafter abbreviated as EL) materials can be made thinner and lighter in weight than conventional CRTs, and are being applied to more and more various uses. Since cellular phones, portable information terminals and the like have obtained Internet connections, there is a drastic increase in amount of graphically displayed information, which raises the demand for display panels to display in color and high definition.
For display devices that are mounted to portable information terminals and the like, being light-weight is considered to be more important. On the cellular phone market, for example, there are products that weigh less than 70 g. Almost all parts employed including individual electronic parts, casings, and batteries are reviewed to make a portable information terminal or the like lighter in weight. However, in order to achieve further weight reduction, display devices, too, have to be lighter in weight.
Display devices in general are made using glass substrates. One way to reduce the weight of display devices is to make the glass substrates thinner. A thinner glass substrate, however, increases the flexibility of a display device in which the substrate is used, and causes the display device to warp easily when a slight force is applied. The warping, in some cases, instantly separates the glass substrate away from a sealing glass sheet that constitutes the display device together with the glass substrate. The separation generates high voltage (electrostatic charge induced by the separation, which can cause a breakdown of the display device or a driving TFT that drives the display device. Decrease in impact resistance is fatal for cellular phones. It is therefore an important subject how a glass substrate should be bonded to a sealing glass sheet in order to achieve weight reduction and impact resistance at the same time. A technique of enclosing a light transmissive portion with glass or transparent resin has been proposed in, for example, Japanese Patent Application Laid-Open No. H10-305620.
Self-luminous display devices for full-color display can be produced from organic EL materials. However, organic EL devices have various confirmed degradation mechanisms, which are obstacles to practical applications and urgent problems to be solved. Heat, light, moisture, oxygen, etc. fasten degradation of an EL layer in an organic EL device.
A common material used for a cathode to cause an organic EL layer to emit light is alkaline metal or alkaline earth metal which is low in work function. It is a known fact that the metal is very reactive with oxygen or water and is easily oxidized. When a cathode which injects electrons into an organic EL layer is oxidized, the material of the cathode loses electrons. Also, an oxide film is formed on the oxidized cathode material. The reduction in number of electrons and influences due to the oxide film are thought to lower the luminance of light emitted from the EL layer.
Dark spots are dot defects in a pixel portion which fail to emit light, and considered a problem that seriously lowers the display quality. Dark spots are progressive defects and are said to increase in number even when EL devices are not in operation if moisture is present. The cause of dark spots is thought to be the oxidization reaction of a cathode formed of alkaline metal or alkaline earth metal. The current countermeasure against dark spots is to enclose an organic EL device and add a desiccating agent thereto.
Those many factors that lead to oxidization, including low heat resistance of organic EL devices and heat which may cause further oxidization, present great obstacles to practical applications of organic EL devices.
Thus, while being very effective for weight reduction and thinning of display devices, employing a thinner glass substrate and sealing glass sheet leaves problems to be solved in order to ensure the reliability of organic EL devices.